MF radars

Medium frequency (MF) radars are typically operated between 2 and 3.5 MHz. With these radars, echoes from the Mesosphere-Lower-Thermosphere can be observed continuously due to partial reflection from relatively sharp boundary between media of different refractive indices. The most common use of MF radars is to measure mesospheric winds in the altitude region of about 60 km to 100 km. There are generally two main methods to measure horizontal winds, which are typically defined by the complexity of the radar and its antenna array:

The Doppler beam swinging (DBS) method, where a narrow antenna beam is steered sequentially into different, typically oblique, directions. From the line-of-sight velocities of individual beam pointing directions, and assuming homogeneity, the mean vertical and horizontal winds are calculated.

The spaced antenna (SA) method uses a vertically pointing antenna beam and at least three non-collinear receiving antennas on reception. From the temporal changes of the diffraction pattern on the ground, the horizontal velocity, shape and coherence of the scattering layers are inferred. Such diffraction pattern is obtained from the cross correlation or cross spectra analysis of signals received at the spaced receiving antennas.

From the derived wind fields e.g. wave parameters and energy dissipation rates can be derived. Additionally, the Saura MF radar allows the estimation of electron density by differential absorption and differential phase measurements.

Example of neutral wind measurements, obtained by the Saura MF radar on June 7th 2010. Positive wind direction describes a flow from south to north (meridional wind). The semidiurnal tide is accentuated in the altitude range from 80 to 100 km.

W. Singer, R. Latteck, D. A. Holdsworth und T. Kristiansen, A new narrow beam MF radar at 3 MHz for studies of the high-latitude middle atmosphere: System description and first results, Proceedings of the 10th international workshop on technical and scientific aspects of MST radars, 2003